1. The Field of the Invention
The present invention relates generally to thermoplastic films. Specifically, the invention relates to stretched thermoplastic films with increased haze or opacity.
2. Background and Relevant Art
Thermoplastic films are a common component in various commercial and consumer products. For example, grocery bags, trash bags, sacks, and packaging materials are products that are commonly made from thermoplastic films. Additionally, feminine hygiene products, baby diapers, adult incontinence products, and many other products include thermoplastic films to one extent or another.
The cost to produce products including thermoplastic film is directly related to the cost of the thermoplastic film. Recently the cost of thermoplastic materials has risen. In response, many attempt to control manufacturing costs by decreasing the amount of thermoplastic material in a given product.
One way manufacturers may attempt to reduce production costs is to stretch the thermoplastic film, thereby increasing its surface area and reducing the amount of thermoplastic film needed to produce a product of a given size. Common directions of stretching include “machine direction” and “transverse direction” stretching. As used herein, the term “machine direction” or “MD” refers to the direction along the length of the film, or in other words, the direction of the film as the film is formed during extrusion and/or coating. As used herein, the term “transverse direction” or “TD” refers to the direction across the film or perpendicular to the machine direction.
Common ways of stretching film in the machine direction include machine direction orientation (“MDO”) and incremental stretching. MDO involves stretching the film between pairs of smooth rollers. Commonly, MDO involves running a film through the nips of sequential pairs of smooth rollers. The first pair of rollers rotates at a speed less than that of the second pair of rollers. The difference in speed of rotation of the pairs of rollers can cause the film between the pairs of rollers to stretch. The ratio of the roller speeds will roughly determine the amount that the film is stretched. For example, if the first pair of rollers is rotating at 100 feet per minute (“fpm”) and the second pair of rollers is rotating at 500 fpm, the rollers will stretch the film to roughly five times its original length. MDO stretches the film continuously in the machine direction and is often used to create an oriented film.
To MDO a film, manufacturers commonly heat the film to an elevated temperature and stretch the film in the machine direction. Commonly, manufacturers will stretch the thermoplastic film between approximately 300 to 500 percent of the film's original length or more.
Incremental stretching of thermoplastic film, on the other hand, typically involves running the film between grooved or toothed rollers. The grooves or teeth on the rollers intermesh and stretch the film as the film passes between the rollers. Incremental stretching can stretch a film in many small increments that are evenly spaced across the film. The depth at which the intermeshing teeth engage can control the degree of stretching. Often, incremental stretching of films is referred to as ring rolling.
Stretched films of reduced thickness can allow manufacturers to use less thermoplastic material to form a product of a given surface area or size. Reducing the gauge (i.e., thickness) of a film; however, can make the film more transparent or translucent. Consumers commonly associate thinner films and/or transparent films with weakness; and thus, may be dissuaded to purchase stretched films. Manufacturers may add pigments and/or additives, such as Ti02 or voiding agents, to add either color and/or opacity to thinner films. Unfortunately, additives, such as Ti02 and voiding agents can be expensive and/or often negatively impact the film strength properties, especially as the additive concentration is increased. Furthermore, even pigmented films commonly become less opaque upon stretching.
Optical measurement affecting opacity include light transmission, haze, and clarity. Light transmission is a measure of how much light is absorbed when passing through a film. Haze measures wide angle light scattering, and is a way of quantifying how well one can see contrast through films. Clarity measures narrow angle light scattering, and is a way of quantifying the ability to resolve detail through a film. ASTM D1003-11 or the Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics describes how haze and other optical properties of films can be measured. ASTM D1003-11 is hereby incorporated by reference in its entirety.
The increasing the transparency or decreasing the opacity of a film upon stretching may dissuade manufacturers to stretch a film or use thinner films despite the potential material savings. For example, one common use of thermoplastic films is as bags for liners in trash or refuse receptacles. Many consumers may prefer opaque and non-transparent trash bags that prevent others (i.e., neighbors) from viewing the contents in the trash bag.
Accordingly, there are a number of considerations to be made in thermoplastic films and manufacturing methods.